2,6-Diamino-3,5-dinitropyridine and diaminodinitrobenzene are intermediates for the preparation of precursors for the manufacture of “rigid rod” polymers used in fabricating films, filaments, and yarns. An example of such rigid rod polymers, poly[pyridobisimidazole-2,6-diyl(2,5-hydroxy-p-phenylene) or poly(2,3,5,6-tetraminopyridine-co-2,5-dihydroxyterephthalate) is described by Sikkema et al. in U.S. Pat. No. 5,674,969. 2,6-Diamino-3,5-dinitropyridine can also be used as an insensitive (safe) explosive and as a multifunctional organic reagent.
2,6-Diamino-3,5-dinitropyridine is prepared by nitration of 2,6-diaminopyridine. The nitration of 2,6-diaminopyridine by reaction with a mixture of nitric acid and sulfuric acid is known from German Patent 3,920,336. The drawback to this process is that it gives a 2,6-diamino-3,5-dinitropyridine yield of not more than 50% of theory.
Sikkema et al. in U.S. Pat. No. 5,945,537 describe an improved process for the conversion of 2,6-diaminopyridine to 2,6-diamino-3,5-dinitropyridine in a single step reaction using oleum (fuming sulfuric acid). The diaminopyridine is added to the oleum, and then concentrated nitric acid is added, and the product isolated. A yield improvement to more than 90% was obtained.
The prior art processes for the production of 2,6-diamino-3,5-dinitropyridine and the corresponding diaminodinitrobenzene are prone to yield product containing variable amounts of heretofore-unidentified impurities. The composition of the impurities and the effect of these impurities on products made using the 2,6-diamino-,5-dinitropyridine and the corresponding diaminodinitrobenzene are unknown.
It is desirable to find a nitration process for the preparation of diaminodinitropyridine and diaminodinitrobenzene that provides a high yield free of impurities. It is also desirable to identify the composition of the impurities. It is also desirable to find a process of making rigid rod polymers having a minimum inherent viscosity of about 25 dL/g. The present invention identifies one of the major impurities and provides such processes.